They might be small and mild mannered, but don't be fooled; the blue-ringed octopus packs a powerful venomous punch. Luckily, says zoologist Lydia Mäthger, they give fair warning to any creature foolish enough to bother them. ‘When they are disturbed, they flash about 60 iridescent blue rings as a warning signal’, she explains. Filming the marine animals, Mäthger found that they can flash their rings in a third of a second. Wondering how they achieve such speedy signalling, she decided to take a closer look at their rings (p. 3752).

Cephalopods (squid, cuttlefish and octopus) are extremely colourful creatures, says Mäthger. ‘They have two ways of changing colour’, she explains. They can use chromatophores, small balloon-like pigment sacs that can be stretched or compressed by muscles, or they can use reflector cells such as iridophores, which are composed of stacked thin plates that reflect light by thin-film interference to generate iridescent colours, much like the spectrum of colours created by the thin surface of a soap bubble.

Working with colleagues at the Marine Biological Laboratory in Woods Hole, Massachusetts, Mäthger first tested whether the iridophores of the blue-ringed octopus can actively produce blue iridescence. ‘Some animals can switch their iridophores on and off using chemical signals,’ explains Mäthger, ‘so we wanted to see if the blue-ringed octopus can too.’ Bathing blue-ringed octopus skin samples in a range of chemicals known to affect chromatophores and iridophores in other cephalopods and fish, she discovered that none of the chemicals had any effect: the structures retained their colour and never switched off. ‘Blue-ringed octopus iridophores are physiologically inert’, concludes Mäthger.

Next, she investigated the optical properties of the iridophores. She suspected that, like those of many other cephalopods, the iridophores of the blue-ringed octopus function as multilayer reflectors, which change colour when viewed at different angles. ‘In squid, for example, some iridophores appear red at normal viewing angles, but change colour from green to blue to UV when viewed at increasingly oblique angles’, explains Mäthger. Examining the light reflected from blue-ringed octopus skin samples from different viewing angles using a spectrometer, she saw that the iridophores displayed the shift to the UV end of the spectrum that is characteristic of multilayer reflectors. But other cephalopods that also have multilayer reflectors are more sluggish signallers, so the blue-ringed octopus must have another trick up its sleeve to produce its fast flashes.

To find out how the octopus pulls this off, Mäthger closely examined the skin structure around the blue rings using different microscopic techniques. She was surprised to discover that the iridophores are tucked into modified skin folds, like pouches, which can be closed by the contraction of muscles that connect the centre of each ring to its rim. When these muscles relax, and other muscles around the perimeter of the ring contract, the pouch opens to expose the iridescent flash. The octopus expands brown chromatophores on either side of the ring to enhance the contrast of the iridescence.

So, its highly elastic, muscular skin is the key to the signalling success of the blue-ringed octopus. ‘This signalling display method has never been seen before’, notes Mäthger. ‘A fast, conspicuous display under muscular control is an advantage to predators, who are warned before attacking a venomous creature, and of course to the octopus itself, as it avoids being eaten.’